The present invention relates to a method of making a tire for vehicle wheels, comprising the steps of: forming an air tube of toroidal conformation, having a transverse profile in the form of a closed ring; covering the air tube with a carcass structure, accomplishment of which involves winding of at least one first thread element around the transverse profile of the air tube so as to form first turns disposed consecutively in side-by-side relationship along the circumferential extension of the air tube to define a first carcass ply integrally covering the air tube; and associating a circumferentially-inextensible belt structure and a tread band circumferentially external to the belt structure with the carcass structure at a radially-outer position.
In accordance with the invention, this method leads to achievement of a tire for vehicle wheels of the type comprising: an air tube or inflatable core of toroidal conformation, having a transverse profile in the form of a closed ring; a carcass structure comprising at least one first carcass ply formed of at least one first thread element wound around the transverse profile of the air tube to form first turns disposed consecutively in side-by-side relationship along the circumferential extension of the air tube; a pair of annular elements for anchoring to a rim, axially spaced apart from each other and extending circumferentially on the air tube surfaces at a radially-inner position; a belt structure circumscribing the carcass structure at a radially-outer position relative to the rotation axis of the tire; and a tread band disposed circumferentially at a radially-outer position relative to the belt structure. Still in accordance with the invention said tire can be coupled with one said rim of the type comprising a central portion for engagement with a hub of a vehicle wheel, and engagement means for said tire; the invention further relates to a wheel consisting of said tire and rim assembly.
It is known that tires for vehicle wheels currently have a toroidal conformation the profile of which seen in cross-section is open on its circumferentially-inner side. In more detail, the tire profile seen in cross-section is delimited between two inner circumferential edges defined by corresponding beads axially spaced apart from each other. An inextensible annular element, usually referred to as a bead core, is integrated into each bead and an elastomer filling, usually obtained by extrusion, is to be circumferentially applied to the outer surface of said bead core.
Turned up around the cross-sectional profile of the bead cores are the flaps of one or more carcass plies. Each carcass ply is made up of a plurality of textile cords disposed parallelly in side-by-side relationship and incorporated in an elastomer layer produced with the aid of appropriate calenders fed from an extrusion die. The rubberized fabric obtained from this step is then transversely cut into pieces of the desired length that are subsequently joined together at the respective side edges, optionally by slight mutual overlapping, in order to make a rubberized ply having transversely-disposed textile cords.
The assembly formed of one or more of said carcass plies and the bead cores with the respective elastomer filling form the so-called carcass structure of a tire.
Applied to the carcass structure, at a circumferentially-outer position, is a circumferentially-inextensible belt structure, comprising one or more superposed strips of rubberized fabric formed of textile and/or metallic cords. Circumferentially superposed on the belt structure is a tread band obtained by extrusion, by means of which the tire gets into contact with the ground.
Beads are so structured and shaped that they interact with respective circumferential portions of a rim with which the tire is associated to ensure a steady connection between these two components of a wheel.
In more detail, coupling between the bead and the corresponding circumferential portion of the rim enables the bead to be constantly pushed against an abutment flange by effect of the tire inflation pressure. In tubeless tires, that is, tires devoid of an air tube, coupling between the bead and the corresponding circumferential rim portion is conceived in such a manner that a hermetic seal of the air contained in the tire is also ensured.
In any case, both in tubeless tires and in tires providing employment of an air tube, the task of counteracting the inner inflating pressure of the tire relies on the rim.
In addition to the above described tires having a transverse profile of open section, use of which is almost universally spread for all types of road vehicles, several types of tire of a transverse sectional profile in the form of a closed ring have been proposed. For example, U.S. Pat. No. 4,232,723 discloses a tire having an air tube and a carcass structure essentially comprising a ribbon ply reinforced with radial threads which extends in the circumferential extension of the tire and is wound by its end flaps around the air tube so as to cover it over the whole transverse sectional profile thereof. A belt structure is interposed between the carcass ply and the air tube, at a circumferentially-outer position relative to the air tube; said belt structure has a substantially flat profile in cross-section and is essentially formed of one or more strips extending circumferentially of the tire and turned up around two inextensible annular elements located at the region usually referred to as the tire shoulder.
Coupling between the tire and rim is obtained by a circumferential ridge of one piece construction with the elastomer material forming the tire sidewalls, and extending at a radially internal and centered position relative to the tire.
Another type of tire having a closed cross-sectional profile is disclosed in U.S. Pat. No. 4,283,366, herein reported as an example of the most relevant state of the art in connection with the present invention.
This tire comprises an air tube having a substantially oval sectional profile, a pair of bead cores axially spaced apart from each other and located directly in contact with the air tube, on the side thereof turned towards the tire axis. The assembly formed of the air tube and bead cores is enclosed in a carcass structure essentially formed of a cord spirally-wound about the cross-sectional profile of the air tube. The cord thus wound forms a plurality of turns disposed consecutively in side-by-side relationship along the circumferential tire extension and oriented in respective planes substantially radial to the rotation axis of the tire itself.
Applied to the air tube at a radially-outer position thereof is a belt comprising one or more circumferentially-inextensible belt strips on which the tread band is superposed.
During the carcass structure formation, as well as during the following steps of the construction process, the air tube in a raw state is maintained to its toroidal conformation by an inner core susceptible of disintegration, said core giving the air tube sufficient consistency so as to enable it to bear any type of handling. This core, essentially consisting of powders aggregated by a binder, is disintegrated after vulcanization of the tire and the remaining powders are removed from the air tube through one or more tire inflating valves.
Use of a sufficiently rigid inner core susceptible of disintegration was made necessary in order to enable winding of the carcass cord around the air tube already shaped in an oval form, because the alternative solution of inflating the air tube to give it the necessary consistency during the tire construction steps inevitably caused the air tube to acquire a cross-section circular shape. To the ends of the present invention it is pointed out that xe2x80x9covalxe2x80x9d and xe2x80x9cellipticxe2x80x9d as herein considered are substantially equivalent terms.
The Applicant has become aware of the fact that laying down of the cord on the air tube having a circular shape in section would involve a variation in the radius of curvature of the cord at the moment of changing the air tube shape from circular to elliptic, as well as a variation in the cord thickness, thereby bringing about an uneven and unbalanced distribution of efforts in the carcass ply.
In accordance with an initial intuition, the Applicant has noticed that thickness control and evenness of the turns disposed on the air tube is, however, less critical for high-performance tires of lowered section, that is, tires in which the ratio between height and width is lower than or equal to 0.70:1.
However, based on this intuition that is only valid for particular items, a solution has been studied that could be appropriate for any type of tire.
In accordance with the present invention, it has been found that manufacture of a tire can be greatly simplified and improved if the carcass structure is made by winding of one or more thread elements directly on the air tube, the latter being shaped in a substantially elliptic conformation and at least partly inflated to such a degree that it reaches a structural strength adapted to prevent it from being subjected to collapsing during working.
In more detail, the invention relates to a method of making a tire for vehicle wheels, wherein before said covering step, the air tube is conveniently shaped by inflation to a preestablished pressure so as to give it the desired structural consistency in the absence of any inner core and maintained to a substantially elliptic configuration during the steps following the shaping or configuration step.
Preferably, the air tube is pre-vulcanized to a vulcanization degree at least equal to 50% and inflated to an actual pressure not exceeding 0.2 bar.
In accordance with a first embodiment, formation of the air tube involves the steps of: injecting an elastomer material into two opposite cavities defined between two respective mold-halves to be moved close to each other and a forming body or former interposed between said mold-halves, to define two halves of said air tube; removing the former from said mold-halves; moving the mold-halves close to each other so as to make the air tube halves fit together at the respective junction end edges; and pre-vulcanizing the air tube within said mold-halves.
According to a possible alternative embodiment, formation of the air tube involves the following steps: introducing a predetermined amount of rubber latex into a cavity of a mold; causing rotation of the mold about at least two orthogonal axes so as to homogeneously distribute the rubber latex on the surfaces of said cavity; and pre-vulcanizing the air tube.
Advantageously, winding up of said at least one thread element is carried out by causing rotation of a reel carrying the thread element about the transverse profile of the air tube, while the air tube itself is caused to substantially rotate about its own geometric axis of rotation.
Preferably, laying down of said at least one first thread element over the air tube takes place at a portion of the air tube which is guided between two mutually-opposite guide collars.
It is also provided that during winding of said at least one first thread element, delivery of said thread element from the reel should be controlled so as to have a linear extent slightly lower than the outer perimetric extension of the air tube transverse profile, for each turn carried out by the reel about the air tube transverse profile, so that tensioning of the thread element does not exceed 2% of elongation.
Winding of said at least one first thread element can be preceded by a step of applying a first layer of raw elastomer material to the air tube.
This application preferentially takes place by winding a first ribbon element of raw elastomer material around the air tube transverse profile, creating turns disposed after each other in side-by-side relationship so as to form a first elastomer layer integrally covering the air tube itself.
According to a preferential alternative solution, formation of the carcass structure involves simultaneous winding up of at least two first thread elements disposed parallelly to define a first ribbon band.
Preferably, the individual thread element or elements forming said first ribbon band are previously incorporated into a layer of raw elastomer material joining them together before the winding step.
Still in accordance with the present invention, accomplishment of the carcass structure further involves application of a pair of annular anchoring elements to the first carcass ply, said anchoring elements being spaced apart from each other and extending circumferentially at a radially-inner position relative to the air tube.
Preferably, application of said annular anchoring elements is carried out by axially moving the latter towards the air tube, which step is followed by a further rolling step of the annular anchoring elements.
Application of the annular anchoring elements may be preceded by a winding step of at least one second ribbon element of raw elastomer material about the transverse profile of the air tube, creating turns disposed after each other in side-by-side relationship so as to form a second elastomer layer covering the first carcass ply substantially integrally.
According to an alternative embodiment of the method in reference, application of the annular anchoring elements takes place concurrently with closure of the carcass structure into a vulcanization mold within which the annular anchoring elements are arranged.
In this case the annular anchoring elements may be made by injection of elastomer material into respective cavities defined within the vulcanization mold.
Alternatively, the annular anchoring elements are made of an injection-molded elastomer material or by extrusion.
A step of incorporating at least one circumferentially-inextensible annular reinforcing insert into each of said annular anchoring elements may be also provided.
This annular reinforcing insert can be incorporated into the respective annular anchoring element during the manufacturing step of the latter, if it is made of an injection-molded elastomer material.
Alternatively, each of said annular reinforcing inserts can be incorporated into the respective annular anchoring element by fitting through a cut arranged in the annular anchoring element.
Preferably, the annular anchoring elements are pre-vulcanized before being utilized for making said tire.
Still in accordance with the invention, manufacture of the carcass structure further involves application of a holding and reinforcing textile structure to an outer surface of each of said annular anchoring elements.
This reinforcing textile structure may comprise at least one ribbon band applied to the respective annular anchoring element before application of the latter to the first carcass ply or it may be applied to the annular anchoring elements subsequently to their being applied to the first carcass ply.
In the last-mentioned case, the holding and reinforcing textile structure is preferably made by winding at least one second thread element around the transverse profile of the air tube to form turns disposed consecutively in side-by-side relationship along the whole circumferential extension of the air tube in order to define a second carcass ply superposed on the first carcass ply.
Winding up of the second thread element can be preceded by a step involving covering of same with a layer of raw elastomer material.
Formation of the second carcass ply may involve a simultaneous winding of two or more of said second thread elements disposed parallelly in side-by-side relationship so as to form a ribbon band and optionally incorporated in a layer of raw elastomer material joining them together before their winding step.
Application of the second carcass ply is preferably followed by a circumferential cutting step, in which said second carcass ply is cut at a radially-internal region with respect to the air tube included between said annular anchoring elements.
The cut flaps of the second carcass ply are then conveniently shaped to make them adhere to the surfaces of the anchoring elements and the first carcass ply.
Preferably, the method in question further comprises a step of applying protection elements of elastomer material to the outer surfaces of the carcass structure.
Application of these protection elements of elastomer material can be advantageously carried out by closure of the carcass structure inside a vulcanization mold within which the protection elements are arranged.
The protection elements can be obtained by injection of elastomer material into respective cavities defined within said vulcanization mold.
Advantageously, manufacture of the belt structure and tread band involves the following steps: applying at least one belt strip around a support drum; and circumferentially applying a tread band of raw elastomer material about said at least one belt strip, so that said belt structure and tread band form an annular element to be coupled with said carcass strip.
The tread band can be advantageously made by spirally winding at least one continuous ribbon element of elastomer material about the belt structure, said ribbon element being produced by extrusion.
A step of arranging a sleeve of raw elastomer material fitted on the support drum before application of said at least one belt strip may be also provided.
The annular element and carcass structure can be vulcanized separately from each other and mutually coupled in a removable manner.
In this case, at least one circumferential hump is preferably formed at a radially-outer position on the carcass structure, before vulcanization of said carcass structure; in addition, in the belt structure, before vulcanization thereof, at least one anchoring seating conforming in shape to said circumferential hump is also defined.
The circumferential hump may be advantageously made by circumferential winding of at least one ribbon element around the carcass structure.
According to a possible alternative embodiment, the annular element and carcass structure are vulcanized simultaneously after their mutual coupling.
According to a further innovatory and advantageous aspect of the invention, during vulcanization the carcass is inflated by admission of a fluid under pressure to the air tube.
The carcass can be also maintained in an inflated condition during a cooling step carried out after said vulcanization step. Vulcanization can be carried out either through heat supply by microwaves or through heating of the mold walls by electromagnetic induction.
In accordance with a possible embodiment, said belt structure can be obtained by winding at least one belt strip directly around the carcass structure.